I do wonder how you guard against the threat of you walking down the street staring at your phone and having someone snatch it out of your hand while it's still unlocked. A saavy thief could then disable the auto-sleep and as long as it doesn't run out of battery they could leave your phone plugged in and hence will never require the passcode again. A few apps (at least on iOS) give you the option to require your passcode again to access them (e.g. if you lock a note), but for the most part an unlocked phone is a treasure trove of personal information for the thief.